Voltage non-uniformity compensation method for high frequency plasma reactor for the treatment of rectangular large area substrates

ABSTRACT

A vacuum vessel and at least two electrodes define an internal process space. At least one power supply is connectable with the electrodes. A substrate holder holds a substrate to be treated in the internal process space. At least one of the electrodes has along a first cross section a concave profile and has along a second cross section a convex profile, the first cross section being parallel to the second cross section. Gas is provided to the space through a gas inlet. Power is provided to the electrodes and the substrate is treated.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a voltage and electrical field non-uniformitycompensation method for large area and/or high frequency plasmareactors. This method is generally applicable to rectangular (or square)large area plasma processing equipment, which is used in—but not limitedto—LCD, Plasma Display and Solar Cell production) or any other reactorusing electromagnetic waves (RF, VHF) for processing.

BACKGROUND OF THE INVENTION

The standard frequency of the radio frequency generators mostly used inthe industry today is 13.56 MHz. This frequency is open for industrialuse by international telecommunication regulations. However, lower andhigher frequencies were discussed and desired from the pioneering daysof plasma capacitor applications. Nowadays, namely in PECVDapplications, (plasma enhanced chemical vapor deposition applications),there is a trend to change to RF frequency values higher than 13.56 MHz,the preferred values being 27.12 MHz and 40.68 MHz (harmonics of 13.56MHz). Higher frequencies allow for higher deposition rates in PECVDprocesses and thus increase productivity and lower product costs.Accordingly, this invention applies to RF frequencies in the range of 1to 100 MHz, but it is mostly relevant to frequencies above 10 MHz.Furthermore, the invention can also be applied up to the microwave rangeof several GHz.

With large area plasma processing equipment, severe problems arise whenthe RF frequency is higher than 13.56 MHz and a large size (largesurface) substrate is used. As described below, the problem addressed bythis invention becomes of real importance when the largest dimension ofthe planar capacitive reactor (the diagonal) is equal or larger than3-5% of the free space wavelength of the RF electric power driving theplasma. Under such circumstances, the reactor size is no longernegligible relative to the free space wavelength of the RFelectromagnetic wave. In such a case, the plasma intensity along thereactor can no longer be uniform. Physically, the origin of such alimitation lies in the fact that the RF wave is distributed according tothe beginning of a “standing wave” spatial oscillation within thereactor. Other non-uniformities can also occur in a reactor, for examplenon-uniformities induced by the reactive gas provided for the plasmaprocess.

U.S. Pat. No. 6,228,438 of the same applicant (hereinafter US '438)describes different ways of solving the standing wave problem, whichleads to voltage non-uniformity distribution over the reactorelectrodes. U.S. Pat. No. 6,631,692 describes a plasma CVD film-formingdevice wherein both electrodes have a concave surface. According to U.S.Pat. No. 6,631,692, this leads to a more uniform plasma, but thisdocument does not address the “standing wave problem” described below.US '438 focuses on the problem of an essentially cylindrically symmetricparallel plate reactor. No known prior art addresses the complexcompensation necessary for the case of a square box shaped reactor withrectangular or square substrates and electrodes.

When a standing wave forms in the cavity, the voltage non-uniformitydistribution can lead to plasma non-uniformity as the plasma issustained in the reactor gap (between the cathode and the anode andabove the substrate). This will lead to non-uniform processing and/ornon-uniform properties of the layers on the substrate, depending on thedesired application (deposition and etching for example). The presentinvention can also be applied to reactors, which do not necessarily useplasma: for example reactors using high frequency electromagnetic wavesfor heating.

To understand and to predict the standing wave problem, experiments havebeen conducted to determine the shape and the intensity of thisnon-uniformity and its dependency on the reactor scale (size) as well asthe excitation frequency. US '438 teaches that the non-uniformityinduced by standing waves depends very closely on the reactor size andthe excitation frequency. Experiments were conducted in two types ofreactors:

-   i) A large cylindrical reactor (with 1 m diameter) where    non-uniformity due to standing waves is very pronounced at high    frequencies (67.8 MHz and 100 MHz) was used for quantitative    studies. FIG. 1-a shows the measured and normalized plasma light    intensity for two extreme Argon plasma conditions (pressure and RF    power) across the reactor diameter. The plasma conditions in this    experiment have been chosen such that all other plasma conditions    would have lead to a light intensity distribution, which is between    the red and green curves. In the absence of plasma, the electrical    field can be calculated to be zero in a distance of 2400 mm from the    central RF injection point. By igniting plasmas under different    power and pressure conditions at 67.8 MHz, it can bee seen in FIG.    1-a, that near zero plasma density already occurs at around 450 mm    distance from the reactor center. This dramatic decrease in    uniformity in presence of the plasma is due to the reduction of the    electromagnetic wavelength causing the standing wave compared to the    vacuum calculation and is due to the effective permittivity of the    plasma and sheath distribution in the inter-electrode gap. By    further increasing the plasma excitation frequency to 100 MHz, it    can be seen that the zero plasma light intensity zone has shifted    from 400 mm in FIG. 1-a to about 300 mm in FIG. 1-b.-   ii) A small rectangular reactor (0.4 m×0.4 m) was used to measure    the ion flux uniformity in Argon plasma. In FIGS. 2-a, 2-b, and 2-c,    the plasma uniformity measured by ion flux uniformity in the plasma    is shown. This ion flux uniformity is directly linked to electron    and ion density uniformity. It should be noted that ion density and    electron density uniformity of the plasma are, at a first order, the    parameters, which are responsible for the plasma processing    uniformity. From these figures it can be seen that the plasma is    relatively uniform at 13.6 MHz and it becomes non-uniform when the    excitation frequency is increased to 60 MHz and 81 MHz. This    non-uniformity is due to the standing wave effect, which is more    pronounced when excitation frequency is increased.    This experimental evidence shows that the plasma non-uniformity,    which is due to standing waves, is dependant on excitation frequency    and reactor size. I other words it is dependant on the scaling    difference between the excitation wavelength and reactor typical    dimension.

Known Problems for the Rectangular Case:

US '438 does not specifically refer to the standing wave problem in thecase of a rectangular reactor and also in the case of very large reactorareas (>1 m² and more typically 3-4 m²) where additional practicalproblems for RF injection points arise. When the reactor area increases,one needs to increase the number of injection points on the excitedelectrode (cathode) in order to distribute the RF current over severalpoints and thus reduce the RF current density and thereby also reducethe failure risks due overheating and thermal impact such as melting,mechanical deformation, fatigue and others more.

For rectangular reactors, which are widely used in PECVD production—forsuch applications and equipment as TFT Displays, Plasma Displays, SolarCells and others—the standing wave form which is created byelectromagnetic propagation has a non-cylindrical symmetry shape (thesolution still has symmetry along the two axes of the electrode) due tothe strong effect of the reactor's corners or the electrode's cornersrespectively. In addition the waveform in the plasma region is dependanton the RF injection geometry, while this injection takes place in thebackside of the electrode. Even if one uses only one RF injection pointwhich is well positioned in the center of the back side of the reactorelectrode, the effect of the corner due to the standing waves has to betaken into account. US '438 describes the shape of the compensatingdielectric layer basically in cylindrical geometry with the thickness ofthe dielectric layer decreasing from the center towards the edges.

This method is strictly speaking optimized for cylindrical geometryreactors but cannot sufficiently compensate the non-uniformity whenusing a rectangular reactor.

SUMMARY OF THE INVENTION

US '438 teaches certain parameter ranges for the reactor size (electrodesize) and/or the excitation frequency, as a “minimum level” (threshold),from which the standing wave starts to give substantial effect on theplasma processing. To quantify this level, one can use a value “r×f”given by the product of size [m] of the electrode and frequency [MHz],whereby “size” is being measured from the centre of the (rectangular orsquare) electrode to the farthest corner. In geometrical terms thisrepresents the radius of an imaginary circle, which circumscribes thecorners of one electrode.

As an example for a “minimum level”, one can define the range of 2-4%deviation of plasma homogeneity. This plasma homogeneity is defined asthe homogeneity of electron density or ion density over the reactorsurface. In some cases, the electron and ion density is not easilymeasured locally; therefore plasma integrated light intensity(luminosity) is measured locally instead, because the light intensity isdirectly correlated to the electron density. Consequently, thehomogeneity of the plasma luminosity is used as a way to measure theplasma homogeneity (FIGS. 2 a and 2 b). On the other hand it isgenerally assumed that the plasma density (electron) is proportional tothe square of the electrode voltage. Therefore, a range of 2-4% inplasma deviation means 1-2% of voltage deviation (half of the plasmadeviation value). It should be emphasized that plasma-processinghomogeneity is not only given by the plasma homogeneity (electron andions) since the chemical equilibrium in the reactor can highlycontribute to the thin film processing homogeneity. Nonetheless, it is arequirement in the design of the plasma processing apparatus to have atleast uniform (homogenous) plasma over the whole surface where theprocess occurs.

In terms of r×f, this threshold is in the range of 5 m·MHz (or 5×10⁶m·Hz). This example corresponds to a frequency of 13.6 MHz and a reactor(electrode) size (radius or half diagonal) of 0.5 m.

On the other hand there is an upper limit for “r×f”, where thetechnology described hereunder finds its limitations. This upper limitis in the range of 50 m·MHz (or 5×10⁷ m·Hz). Furthermore there is alsoan upper limit for the excitation frequency, which today is in the rangeof 2500 MHz.

A vacuum treatment apparatus according to the invention thereforecomprises a vacuum vessel, at least two electrodes defining an internalprocess space and at least one power supply connectable with saidelectrodes. A substrate holder for a substrate to be treated is providedfor in the internal process space and gas inlet means, e. g. accordingto the shower head principle. At least one of said electrodes has alonga first cross section a concave profile and has along a second section aconvex profile, the first cross section being parallel to the secondcross section. This can be found described in detail in the sectionbelow and with aid of some figures.

In one embodiment two of said electrodes in the vessel each have along afirst cross section a concave profile and along a second section aconvex profile, the first cross section being parallel to the secondcross section. Advantageously these electrodes may be machined to beshaped equally. The power supply may be a radiofrequency power supplyfor frequencies of 13.56 MHz or higher, which in a preferred embodimentis connectable with at least one of these electrodes at least twoconnection points. The space between said substrate and one of theelectrodes can according to a further embodiment of the invention be atleast partially furnished with a corrective dielectric layer, which can(further embodiment) complement the shape of the electrode and beadapted to hold the substrate.

In order to make use of the invention a method for treating at least oneflat substrate in such a vacuum treatment apparatus will comprise thesteps of introducing said substrate in the internal process space,providing gas to the internal process space via gas inlet means,applying power to the internal process space via said electrodes andtreating said substrate, whereby comprises e. g. heating, coating oretching.

DESCRIPTION OF THE DRAWINGS

FIG. 1-a: Plasma light intensity distribution over reactor diameter for67.8 MHz excitation frequency for two extreme Argon plasma conditions.All other conditions lead to a light intensity distribution in-betweenthese two curves. The plasma density variations induced by the standingwave effect is clear as it leads to a nearly zero density plasma forradius=400 mm at 67.8 MHz. In the experimental conditions and withoutplasma, the decrease to zero is predicted to be at r=2400 mm. This meansthat the presence of the plasma amplifies the standing wave effect,leading to a decrease of the field at a much smaller radius (r˜450 mm).

FIG. 1-b: Plasma light intensity distribution over reactor diameter for100 MHz excitation frequency for various plasma discharge conditions atpressure levels between 10 and 500 mbar (in the PECVD range) and priorart flat electrodes. Note the shift in position of the minimum lightintensity between 67.8 MHz (about 450 mm in FIG. 2-a) and 100 MHz (about300 mm in FIG. 2-b).

FIGS. 2-a, 2-b, and 2-c: Plasma Ion flux (Ji) uniformity profile for 0.4m×0.4 m reactor measured by electrical probes (64=8×8 probes) for 3different excitation frequencies. 2 a: plasma ignited at 13.6 MHz, 2 b:plasma ignited at 60 MHz, and 2 c: plasma ignited at 81.4 MHz.

FIG. 3-a: 3-D shape of the electrode for a rectangular reactor. Theglass is positioned on the CDL. The CDL is then between the glass andthe electrode. Note that the minimum CDL thickness can be larger thanzero, which means that the glass substrate does not necessarily liedirectly on the electrode.

FIG. 3-b: 3-D shape of the electrode according to the invention. The CDLis the complementary part of the electrode (i.e. the “negative” of theelectrode).

FIG. 3-c: when moving along a line from F to C in FIG. 3-b, the surfaceof the electrode has a generally convex shape (in AFD), which graduallychanges to become concave (in ECH). The same is true for allcorresponding movements form the half-length points (E, F, G, H) towardsthe center C. In FIG. 3-c it can also be seen that the electrode isthickest in E and H, thinner again in F and G, thinner again in A, B,and D and thinnest in C.

FIG. 3-d: another view of the electrode shape.

FIG. 3-e: shows the difference between the electrode shape of prior art(light gray) and the present invention (dark grey). The Gaussian curveof US '438 intersects with the edges to a concave curve and not to aconvex one as in the present invention. Thereby the plasmanon-uniformity is not compensated for in the corners of the reactor.

FIG. 4: variant 1 for a reactor and a compensation configuration.

FIG. 5: variant 2 for a reactor and a compensation configuration.

FIG. 6: variant 3 for a reactor and a compensation configuration. Inthis case, it is important, that the substrate makes contact with themetallic part of the electrode; otherwise the plasma will fill the spacein between and destroy the compensation effect. The drawing exaggeratesthe distance at the edge.

FIG. 7: variant 4 for a reactor and a compensation configuration

FIG. 8 a: contour line of the CDL shape (top view). These contour linesdescribe how the electrode is to be machined.

FIG. 8 b: side view of the CDL and electrode assembly. The number ofsteps is chosen to minimize the difference between the ideal shape andthe approximated step shape and machining cost.

SOLUTION ACCORDING TO THE INVENTION

The present invention introduces a compensation scheme for the standingwave problem in rectangular or square reactors. This compensation isbased on a Compensating Dielectric Layer (hereinafter CDL or “Lens”).The CDL layer can be of any dielectric material including vacuum, gas,liquids or solids. The CDL has a flat planar surface towards the plasmaside and it has a complexly curved surface towards the electrode side.

If this complexly curved surface of the CDL is considered geometricallypositive, the surface of the electrode forms the correspondinggeometrical negative (FIG. 3-a). In order to accommodate the CDL layerof the present invention, the electrode must be shaped accordingly. Inthe following, the curved surface of the electrode (which is essentiallythe same but inverted surface as the CDL) shall be described.

In opposition to the rather smooth, concave and regularly shaped lensvalid for cylindrically symmetric reactor (as described in US. '438), wehave found that the best design for a compensation length in rectangulargeometry is not as simple. We describe this optimal geometry below.

As a reference, a plane of maximum electrode thickness is introducedwhich goes through points E and H in FIG. 3-b and lies in parallel tothe XY plane. Depending on which dielectric material is being used, thisdoes not mean that the CDL thickness is zero at points H and E. (FIGS.3-a, 3-b, and 3-d). In FIG. 3-b, four electrode thickness profiles aredescribed: sections AFD and ECH are parallel to the YZ plane (theshorter dimension) and sections AEB and FCG are in the XZ plane (thelonger dimension). C lies in the center of the electrode and E, F, G andH each describe the half-lengths in their respective dimensions.

The thickness of the CDL and thus the decrease from the maximumelectrode thickness is largest in the centre of the electrode (point C)and the electrode is thinnest in C accordingly. The electrode isthickest in E and H (the half lengths of the longer dimension). Thethickness of the electrode thus decreases successively from E&H to F&Gto A&B&D to C.

FIG. 3-c further describes the necessary shape of the rectangularelectrode to accommodate the CDL: when moving along a line from F to C,the surface of the electrode has a generally convex shape (in AFD),which gradually changes to become concave (in ECH). The same is true forall corresponding movements form the half-length points (E, F, G, H)towards the center C. In FIG. 3-c it can also be seen that the electrodeis thickest in E and H, thinner again in F and G, thinner again in A, B,and D and thinnest in C.

For a square reactor, where CF is of equal length than CE, and AD is ofequal length than AB, it can easily be concluded that the electrodethickness is equal in E, F, G and H and larger than in A, B and D.

Typical CDL dimensions could be:

-   From 0 to 5 m in X, Y dimension (length, width).-   Typically several millimeters in Z dimension (but may also be up    several centimeters with large devices and high frequencies).    A numerical example is shown in FIG. 3-c: AB=2.2 m, AD=2 m, the    maximum gap (d_(max)) is in the range of 3.2 mm for a CDL made only    with vacuum and an excitation frequency of 27 MHz.

FIG. 3-e shows the difference between simple adaptation of prior art (US'438) and the present invention: the shape of the CDL is by no means anadaptation of a Gaussian curve (light gray) to a rectangular electrode,but according to the present invention a carefully designed shape(black) which takes special measures to compensate for plasmanon-uniformity in the corners of a rectangular reactor.

EMBODIMENTS OF THE INVENTION

The present invention focuses on a solution with a compensated bottomelectrode and using vacuum (or gas) as a dielectric. Other embodimentsfor the dielectric compensation can also be used, as well as differentvariants for the reactor configuration:

-   Two reactor configurations can be used: I) the CDL can be used for    the bottom electrode which commonly is used as a substrate    holder, II) the CDL can be used in the top electrode which is    commonly used as a so called gas shower head for plasma processing    purposes (PECVD, PVD, Etching and other such processes).

It can be useful to add the case with a lens on both electrodes. In thatcase, each lens thickness would be half the thickness of a lens in asingle electrode. This case can also offer some advantage in terms ofsymmetry of the reactor (both electrodes are the same). It also permitsto decrease the amplitude of the radial component of the electric field,which is a consequence of the electrode shape. This radial electricfield may be the limiting factor in terms of uniformity when the lensbecomes thick or when the plasma becomes very conductive (high densityplasma at low pressure).

Also, two main CDL embodiments can be used: I) the dielectric can bevacuum or gas (with a relative dielectric constant ε_(r) near to 1), orII) it can be filled with an appropriate dielectric material (with ε_(r)larger than 1) such as alumina, zirconia, quartz or any other materialwhich fulfills the thermal and chemical compatibility specifications ofthe targeted processing.

By combining the above-mentioned variants, one can in summary end withfour main embodiments:

-   Variant 1 (FIG. 4): the CDL is placed in or on the top electrode,    and vacuum is used as main dielectric. In this case, a dielectric    plate can be used on the top electrode in order to maintain plasma    with a uniform gap (plasma thickness in z-axis). The process gas is    flowing from the upper electrode through a given distribution means    (shower head) and then flows through the dielectric plate where    appropriate holes are distributed to let the gas pass towards the    plasma zone.-   Variant 2 (FIG. 5): the CDL is placed in or on the top electrode and    a dielectric material (with ε_(r)>1) is used. This dielectric    material can be porous to let the gas pass through to the plasma    zone or a distribution of holes can be machined into it for the same    purpose.-   Variant 3 (FIG. 6): the CDL is placed in the region of the bottom    electrode. Vacuum is used as a dielectric and the substrate is used    as a way to maintain the plasma gap constant across the whole    reactor volume. The substrate commonly is held over the dielectric    and rests on pins distributed such that they keep the substrate in a    substantially flat position. In order to adequately support the    substrate, a supporting pin distribution with pin-to-pin distances    in the range of 100 mm is preferred to maintain the flatness of a    glass substrate of typically 0.7 mm thickness at temperatures of    about 300° C. The supporting pins are designed to have as little    influence on the behavior of the plasma as possible, since this    leads to a direct perturbation on the film being processed on the    substrate, effecting local defects of the film (thickness,    electrical properties). Its has been found that using thin (slim)    supporting pins (with typically r<2 mm) can reduce this influence to    a level of <2%. (See also DE10134513 A1)-   Variant 4 (FIG. 7): the CDL is shaped in or on the bottom electrode    and a dielectric material (with ε_(r)>1) is used.-   Variant 5: the CDL is shaped in or on the bottom electrode and a    combination of dielectric materials (ε_(r)=1 and ε_(r)>1) is used.

As is obvious to those skilled in the art, almost all of these variantscan again be combined with one another. Since the shape of the CDL israther complex, the same holds also for the electrode, whichaccommodates the CDL. In practice, it is possible to machine theelectrode, which will contain the CDL and/or the dielectric material ofthe CDL using an approximation of more simple steps of a given height inorder to save machining cost. The number of steps necessary to correctlydefine the electrode shape is given by the fact that these steps shallnot influence the plasma process. In PECVD, the practical step heightshall not exceed 0.1 mm. Accordingly for a 3 mm CDL; one should machineat least 30 steps of 0.1 mm height.

In fact we can be more general for the number of steps. We can estimatethe number of steps needed in terms of the desired uniformity. If Uflatis the electric field uniformity for the flat electrode case, then anestimation of the uniformity for n steps is given by Uflat/(nsteps+1)i.e. if Uflat=10% then 9 steps give a 1% uniformity.

In FIG. 8-a, an example of contour lines is presented. These contourlines can be used to define the position of the steps to be machined. Itis thus possible to simplify the manufacturing. A corresponding crosssection is presented in FIG. 8-b.

FURTHER ADVANTAGES OF THE INVENTION

The CDL layer and the corresponding electrode according to the inventioncan also be used for other processes, which do not necessarily implyplasma. It can be used for chambers that utilize electromagnetic wavesfor heating (or for drying). In such an application, the process couldbe heating and the compensation of electric field non-uniformity by theCDL will help to achieve uniform temperature distribution.

By achieving a uniform plasma density with the present invention, a verycritical parameter for layers used in LCD, plasma display orsemiconductor applications is the so-called wet-etch rate uniformity,which closely corresponds with the stoichiometry of a layer: the presentinvention leads to a far more uniform wet etch rate of layers across thesubstrate area, therefore allowing for a reduction of the total layerthickness and thus saving cost in the PECVD deposition as well as in thedry or wet etching process.

By obtaining a more uniform plasma distribution, the present inventionallows for a whole string of advantages which are a consequence ofincreased layer uniformity and layer thickness uniformity: a higherdoping uniformity of semi conducting layers across the substrate areacan be achieved. For example in LCD technology, more uniform layers areetched more evenly which can be an advantage for back channel etchoptimization and thus leads to thinner doped layers, thinner HDR layers,shorter etching times, better mobility, and generally lower productioncost.

In addition, a critical problem in PECVD processes is the presence andarrangement of pins that are used to lift up the substrate from thebottom electrode, the transport pins as opposed to the supporting pinsmentioned above. These transport pins lift the substrate at differentstages of the process (for example during loading and unloading of thesubstrate) and thus need to be designed robustly so that they will notbreak. The presence of those transport pins in standard reactors islikely to cause film growth perturbation (by temperature non-uniformity,electrostatic field perturbation) resulting in a non-uniform etch rate.This effect is particularly disturbing, if the pin is located in or nearthe active area of a device, for example in the active area of a LCD TFTarray. With this invention, the dielectric lens below the substrategreatly reduces the impact of the transport pin perturbation. Thepresent invention thus allows the use of pins with larger diameters forthe implementation of more robust pin designs. It shall be underlinedthat static supporting pins (as shown in FIG. 6) can be made smallenough to reduce the process perturbation to a non-significant level.

1. A vacuum treatment apparatus for treating a substrate, comprising anelectrode having a surface with peripheral edges that define arectangular or square shape when viewed from above, said surface havinga generally convex profile along a respective edge cross section takenalong each said peripheral edge, which profile gradually changes onapproaching the center of said electrode to become a concave profilealong a center cross section that is parallel to the respective edgecross section and is taken through the center of said surface.
 2. Avacuum treatment apparatus for treating a substrate, comprising at leasttwo electrodes defining a process space, two of said electrodes eachhaving a first and a second surface, respectively, each with peripheraledges that define a rectangular or square shape when the respectivesurface is viewed from above, each said surface having a generallyconvex profile along a respective edge cross section taken along eachsaid peripheral edge of each said surface, which profile graduallychanges on approaching the center of each said surface to become aconcave profile along a center cross section that is parallel to therespective edge cross section of the respective surface and is takenthrough the center thereof.
 3. A vacuum treatment apparatus according toclaim 2, wherein said electrodes are shaped equally.
 4. A vacuumtreatment apparatus according to claim 1, further comprising aradiofrequency power supply capable to supply frequencies of 13.56 MHzor higher connected with said electrode.
 5. A vacuum treatment apparatusaccording to claim 2, wherein at least one of said electrodes isconnectable with a power supply at at least two connection points.
 6. Avacuum treatment apparatus according to claim 1, further comprising asubstrate positioned adjacent and spaced from said electrode fortreatment of said substrate, wherein the space between said substrateand the electrode is at least partially furnished with a correctivedielectric layer.
 7. A vacuum treatment apparatus according to claim 6,wherein one surface of the corrective dielectric layer complements theshape of said surface of said electrode.
 8. A vacuum treatment apparatusaccording to claim 6, wherein one surface of the corrective dielectriclayer is adapted to hold the substrate.
 9. A vacuum treatment apparatusaccording to claim 6, wherein the corrective dielectric layer comprisesvacuum, gas, alumina, zirconia or quartz.
 10. A vacuum treatmentapparatus according to claim 1, further comprising a gas inlet systemcomprising a set of holes in said electrode and/or in a correctivedielectric layer positioned adjacent said substrate to provide gases toan internal process space of said apparatus where a substrate is to betreated.
 11. A vacuum treatment apparatus according to claim 6, furthercomprising a gas inlet system comprising a set of holes in saidelectrode and/or said corrective dielectric layer to provide gases to aninternal process space where said substrate is positioned.
 12. A methodfor treating at least one flat substrate in a vacuum treatmentapparatus, comprising the steps of: introducing said substrate in aninternal process space defined between at least two electrodes, one ofwhich being the electrode described in claim 1, providing gas to theinternal process space via gas inlet means, applying power to theinternal process space via said electrodes, and treating said substrate.13. A method according to claim 12, wherein treatment comprises one ofheating, coating or etching.
 14. A method according to claim 12, whereinthe substrate comprises one of a glass substrate, a flat panel display,a semiconductor substrate.